What blood investigations are needed for a child with failure to thrive and chronic diarrhea?

Blood Investigations for Child with Failure to Thrive and Chronic Diarrhea

Begin with a focused panel of screening blood tests to exclude common treatable causes including celiac disease, anemia, and malabsorption, while avoiding extensive laboratory evaluation unless specific clinical features suggest organic disease. 1, 2

Initial Screening Blood Tests (First-Line)

Complete Blood Count (CBC) with Differential

• Assess for anemia (iron deficiency from malabsorption or chronic blood loss) 1
• Evaluate for thrombocytopenia and leukopenia (seen in storage diseases like ASMD) 3
• Check for eosinophilia (suggests food protein-induced enterocolitis syndrome or parasitic infection) 4
• Identify thrombocytosis (reported in 65% of acute FPIES cases) 4

Celiac Disease Serology

• Tissue transglutaminase IgA (TG2-IgA) is the primary screening test with 98-100% accuracy when malabsorption symptoms including failure-to-thrive are present 5
• Total IgA level must be measured simultaneously to exclude IgA deficiency (which would cause false-negative TG2-IgA) 5
• If IgA deficient, obtain IgG-based tests (deamidated gliadin peptide IgG or TG2-IgG) 5

Basic Metabolic Panel

• Serum sodium and electrolytes to detect hypernatremic dehydration or metabolic acidosis 4, 6
• Blood glucose to exclude hyperglycemia with osmotic diuresis 6

Liver Function Tests

• Elevated transaminases occur in celiac disease, ASMD, and other storage disorders 3
• Albumin level assesses for hypoalbuminemia from protein-losing enteropathy 3

Additional First-Line Tests Based on Clinical Context

• Thyroid function tests (TSH, free T4) if autoimmune thyroid disease suspected, as celiac disease co-occurs with autoimmune conditions 5
• Fasting lipid panel if hepatosplenomegaly present (dyslipidemia with decreased HDL and increased triglycerides suggests ASMD) 3

Second-Line Blood Tests (When First-Line Normal but Symptoms Persist)

Stool Studies Should Precede Additional Blood Work

• Stool culture for bacterial pathogens (Campylobacter, Salmonella, Shigella) as chronic Campylobacter can cause failure to thrive 7
• Fecal calprotectin to exclude inflammatory bowel disease 1
• Fecal elastase for pancreatic insufficiency 1

If Malabsorption Suspected Despite Normal Initial Tests

• Fat-soluble vitamin levels (A, D, E, K) indicate fat malabsorption 1
• Prothrombin time/INR (vitamin K deficiency from malabsorption) 1
• Iron studies, ferritin, B12, and folate (deficiencies suggest small bowel disease) 1

If Storage Disease or Metabolic Disorder Suspected

• Enzyme assay for acid sphingomyelinase (ASM) activity if hepatosplenomegaly, dyslipidemia, and chronic diarrhea present 3
• Consider genetic testing early if congenital diarrhea and enteropathy (CODE) suspected, as whole-exome sequencing can expedite diagnosis 8

Critical Clinical Caveats

Avoid Extensive Laboratory Evaluation in Most Cases

• Laboratory testing yields positive results in only 3.2% of children with failure to thrive in gastroenterology clinics 2
• 89% of children with failure to thrive have nonorganic etiologies requiring behavioral and nutritional interventions rather than extensive testing 2
• The majority of children (56.4%) undergo laboratory evaluation, but this should be judiciously performed based on specific clinical indicators 2

Red Flags Requiring Expanded Workup

• Bloody diarrhea with mucus suggests invasive bacterial infection requiring immediate stool culture 9, 10
• Hepatosplenomegaly with chronic diarrhea at 6 months warrants evaluation for storage diseases (ASMD) with enzyme assays and genetic testing 3
• Persistent symptoms despite adequate caloric intake and behavioral interventions justify second-line testing 2
• Eastern European Jewish ancestry with hepatosplenomegaly should prompt early ASM enzyme assay 3

Age-Specific Considerations

• Infants under 2 years with chronic diarrhea: prioritize celiac serology, stool studies for infection, and consider food protein-induced enterocolitis syndrome 4, 8
• Normal infants may shift to lower growth percentiles in first 2 years without representing true failure to thrive 11
• Premature infants and those with Down syndrome follow different growth patterns and should not be diagnosed with failure to thrive based on standard curves alone 11

When to Consider Genetic Testing Early

• Diarrhea from birth with severe malabsorption unresponsive to dietary modifications suggests congenital enteropathy requiring whole-exome or genome sequencing 8, 12
• Consanguineous parents or affected siblings increase likelihood of genetic disorder 12
• Villus atrophy with crypt hypoplasia on biopsy without inflammatory infiltrate suggests familial enteropathy 12